Heat shrinkable plastic tubing is used in a variety of industries and can be made from many types of plastics or polymeric materials, including elastomers. In some instances, stock tubing is heated to a specified temperature and fluid, such as compressed dry air is pumped into the tubing to increase internal pressure within the tubing. The combination of heat and pressure expands the tubing to a desired diameter. Next, the expanded tubing is cooled while pressure is maintained within the tubing. Once cooled, the tubing remains expanded to the desired diameter. If the tubing is then heated at or above the expansion temperature, it will shrink back to its original diameter.
Most shrinkable plastic tubing is manufactured from polyolefin-based polymers and is used to cover electrical wires, forming an insulating layer. Generally, heat-shrink tubing used in common industrial applications is relatively easy to fabricate from a cross-linked polymer or any other suitable thermoplastic polymers and elastomers that experience strain hardening when stretched or expanded. At high strain, strain hardening takes place within a thermoplastic elastomer (TPE) and converts the TPE from elastic behavior to a more leathery or stiff behavior such that some elasticity is lost in exchange for an increase in tensile strength and modulus.
Many polymeric, elastomeric, and fluoropolymeric materials are used to manufacture heat shrinkable tubing because these materials can be expanded without requiring any external diameter constraint during the manufacturing process. Exemplary materials include, but are not limited to polyester, polyethylene (PE), polyvinylidene fluoride (PVDF), as well as other similar materials that would be known to one of ordinary skill in the art with the present disclosure before them.
A variant of free expanding processes involves cross-linking of polymeric chains of the tubing before expansion. In some instances, the tubing may be subjected to an electron beam which breaks continuous polymeric string bonds. These bonds may then reform and cross-link by connecting with other strings. This cross-linking allows the material to be stretched up to a point (e.g., yield stress point) and then gain more strength via strain hardening than a non-cross linked amorphous material. More specifically, some TPE materials may behave as elastomers until the yield stress has been reached via stretching or expanding of the material. If stretched or expanded past the yield stress the TPE material may undergo plastic flow, part of which may be viscoelastic and part will be permanent set (e.g., strain hardening). Material that has been cross-linked can be stretched further to produce tubing having relatively thin walls. Advantageously, thin walled tubing may be used to insulate aircraft cable where weight reduction is beneficial.
Medical catheters also make use of heat shrink tubing to form hollow and flexible shafts that may be guided through arteries, such as a femoral leg artery. The catheter can pass through the heart and then to other areas of the body via arteries or vessels of suitable diameter. Thus, even very small arteries in the brain can be accessed by means of very small catheters to perform neurovascular procedures.
Small diameter catheters may be manufactured by embedding a wire mesh between layers of tubing material. The resultant product remains flexible but can be navigated around curves and bends within the vascular system. This bending flexibility and torsional stiffness can be obtained in a very small diameter catheter shaft if the walls are sufficiently thin.
Processes for making small diameter catheters may include using outer fluoropolymer tubing which may exert compressive forces on the outer layer of the catheter, when the fluoropolymer tubing is heat shrunk. That is, the catheter may be comprised of one or more tubular layers of material. Shrinkable outer fluoropolymer tubing may be used to compress the layers together.
Fluoropolymers, such as polytetrafluoroethylene (PTFE) melt at a much higher temperature than the typical plastic used for the tubing layers used to manufacture the catheter. Thus, heat applied to the layers of tubing during the shrinking process may occur at temperatures which are high enough to almost melt one or more of the layers, allowing the tubular layers to melt and integrate with a wire layer. The integration of the wire layer allows for production of catheters having relatively thin walls that maintain strength and flexibility. In some instances, the fluoropolymer layer is cut off and discarded after the catheter outer layer is finished.
Other common fluoropolymer materials include, but are not limited to PTFE and fluorinated ethylene propylene (FEP). While these fluoropolymers melt at higher temperatures, the fluoropolymer material may each behave differently during expansion and contraction (e.g., shrinking). Manufacturing shrinkable tubing from fluoropolymer materials is often difficult because these materials lose strength when expanded and must be constrained during the expansion and heating process. If not constrained, these materials may expand to failure.
Common process method for constraining FEP during manufacture require passing of the material through a glass tube that is heated and injected with a high-temperature lubricant such as silicone oil. The silicone oil prevents the tubing from sticking to the glass tube. Deleteriously, this process often results in deposition of silicone oil residue on the expanded tubing. Silicone oil is hydrophobic and spreads across any contacting surface. Moreover, silicone oil is difficult to remediate and may contaminate any area where it is present. Thus, in catheter manufacturing systems, any amount of silicone oil can spread through the process environment and is very difficult to remove. Silicone oil may repel water (as well as paint) and may also interfere with gluing or fusing processes that are used to make catheters.